In the field of underground pipes and pipelines it has become common to retrofit such pipelines with plastic liners to extend the usable life of these pipelines since the service life of metal pipes may be drastically shortened when such pipes are used to transport corrosive materials or abrasive materials such as slurries in which solid materials are mixed with liquids to allow transport via pipe. A flanged joint in such a lined pipeline is similar to the flanged joint in a steel pipeline. Each of the mating ends of the metal pipeline is fitted with a metal flange capable of being bolted to a similar flange on the opposing mating pipe end. A similar plastic liner flange, usually integral with a short or "stub" end of plastic liner material is attached to each respective mating end of the plastic liner to provide each liner end with a plastic annular flange. In such a typical joint in the prior art as shown in FIG. 7 each plastic flange P is seated snugly against its respective metallic or steel flange S. Assembly of the joint involves bolting the steel flanges together using bolt retaining holes H to uniformly compress the plastic flanges between the steel flanges. A metal retaining band or collar R is provided around the periphery of the plastic flanges and between the metal flanges to radially confine the plastic flanges and provide a precisely sized spacer between the steel flange faces. Such a joint creates a secure seal and may be constructed to be capable of retaining pressures within a pipeline of thousands of pounds per square inch.
However, a common characteristic of plastics, such as high molecular weight polyethylene, which may be used to line a pipeline is that of "cold flow," or "creep", when the material is subjected to compression. The tendency of the material is to plastically deform by moving toward an area of little resistance. As can be most readily understood by viewing the pipeline joint in cross section, each of the mating plastic liner flanges is typically constrained in the directions parallel to the pipe section on one side by abutting the mating plastic flange and on the other side by abutting the respective opposed steel flange. The outer edge of the flanges is constrained against outward radial movement by a metal retaining band or collar R or compressible wound metallic gasket such as that well known in the industry and sold under the trademark as a "Flexitallic" gasket. However there is virtually no resistance to inward radial flow of the liner material other than that inherent in the properties of the plastic material itself. Accordingly, it is not uncommon for "cold flow" of the respective plastic flanges to create irregularities or an annular ridge on the otherwise smooth cylindrical interior surface W of the pipe liner at area adjacent the joint interface face I between the two plastic flanges as is shown in FIG. 7. The ridge, while slight, interrupts the continuous smooth interior surface of the pipe lining and may have the effect of disrupting smooth fluid flow through the pipeline by creating turbulence or "eddy currents" in the area of the pipeline joint. This turbulence is generally undesirable. Where the pipeline may be used to transport abrasives material such as a slurry mixture of liquid and solids, such turbulence can create a much higher degree of abrasive wear of the pipe liner, resulting in a shortened service life of the liner before replacement may be necessary. Obviously then, the elimination or minimization of the causes of any such turbulence is highly desirable for a pipeline used for slurry transportation. By providing a flanged joint in which cold flow resulting from compression of the pipe liner flanges is isolated to a portion of the flange which is not at the joint interface on the interior surface of the liner, the smooth and continuous cylindrical interior surface the liner may be maintained thereby avoiding the difficulties caused by turbulent flow within the pipe liner